In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase when heated to a melting temperature. The printhead unit ejects melted ink supplied to the unit onto media or an imaging member. Once the ink is ejected onto media, the ink droplets quickly solidify.
The media used in both direct and offset printers may be in web form. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roll. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web.
Moving the web through the media path in a controlled manner presents challenges to web printing systems. As the media web moves through various portions of the media path, one or more of the rollers apply tension to the web. An appropriate level of tension between the media web enables the media web to engage the rollers in the media path via friction without slipping. During operation, however, one or more rollers that contact the media web may slip relative to the motion of the web, resulting in a drop in the tension of the media web. Various causes of the drop in tension include excess slack introduced by a media web winder or rewinder, variations in the rotational rates of two or more drive rollers that are positioned on the media path, and a loss of friction between the media web and a roller due to oil or other substance on the web that reduces the coefficient of friction between the media web and the roller.
If the web slips when engaged with one or more rollers in the media path, the position of the media web with respect to the printheads is affected and errors in images formed on the media web may occur. Media slippage may produce positional errors based on calculations of web velocity because the actual velocity of the web and the web velocity identified with respect to the angular velocity of the rollers differ. These positional errors may adversely impact the effectiveness of registration techniques that coordinate the operation of multiple printheads to form ink images on the media web. When a media web slips over rollers in the media path, one solution known to the art increases a normal force between one or more rollers and the media web to reduce or eliminate the slip and restore tension to the web. The increased normal force applied to the media web may be great enough to distort or break the media web. Thus, improvements in operating continuous web printing systems to reduce slip on the media web while maintaining the normal force that is applied to the media web within an operating range would be beneficial.